Re: two nasty schemata, union types and surrogate keys

Brian wrote:
> ... According to> Date in /An Introduction to Database Systems, Eighth Edition/, page> 161: 'the Closed World Assumption (also known as the Closed World> Interpretation) says that if an otherwise valid tuple--that is, one> that conforms to the relvar heading--does /not/ appear in the body of> the relvar, then we can assume the corresponding proposition is> false. In other words, the body of the relvar at any given time> contains /all/ and /only/ the tuples that correspond to true> propositions at that time.' So the closed world assumption tells us> that what is actually in the database is supposed to be true, while> what is not is supposed to be false.> ...

Regarding the actual quote, I've long taken it to mean also that if a
relvar's complement were recorded and an otherwise valid tuple did not
appear in the complement, then it must appear in the body of the relvar.
Am I right?

Also, do views/derived relvars, eg., joins and unions, have complements
that could theoretically be recorded?
Received on Fri Sep 25 2009 - 18:45:51 CDT